By using finned tubes, is the heat transfer coefficient on the fin side increased?

Using finned tubes to increase the heat exchange area is a method for enhancing heat transfer; it raises the heat transfer coefficient on the side with a lower coefficient (compared to that of plain tubes), thereby increasing the overall heat exchange area. However, when performing calculations, the heat exchange area should be calculated based on that of plain tubes. This is my personal understanding

It will not increase the heat transfer coefficient, as the heat transfer through the film on the outer surface does not increase under the same operating conditions; however, it increases the thermal resistance at the contact between the fins and the tubes. The heat transfer coefficient will only decrease.

The heat transfer coefficient will only decrease; it’s simple – generally, using fins to increase the area doesn’t yield as good results as direct heat transfer between the two media. However, if we consider the area of the bare tubes, it seems that the heat transfer coefficient increases significantly.

The fin temperature decreases gradually with height, indicating that the difference between the fin temperature and the temperature of the surrounding fluid is shrinking, and thus the heat transfer rate per unit area is decreasing. As a result, the effectiveness of the fin surface area in enhancing heat transfer decreases. The higher the fins, the smaller the “contribution” of the increased area to heat transfer.

Finned tubes only increase the effective heat transfer area of the fins, without increasing the heat transfer coefficient. However, for practical calculation purposes, we often convert it into a heat transfer coefficient relative to the base diameter of the light tube, which makes it appear as though the heat transfer coefficient has increased.

Whether it is a heat exchange tube with external fins or one with internal fins, heat exchange between the media on both sides always takes place through the wall of the main tube. As the friend on floor 6 said, the end of the external fins is located far from the main tube; therefore, the temperature difference across those fins is low, and the heat exchange efficiency is weakest at this location. I think this makes it easier for everyone to understand. The same principle applies to the inward-fanning finned heat exchange tubes, with the center of the tube being the weak area. Therefore, whether it is external finned heat exchange tubes or internal finned heat exchange tubes, when striving for a higher fin ratio, it is necessary to consider the optimal structure of such finned tubes in order to achieve the best finning efficiency; this will enable them to offer a greater advantage in enhancing heat transfer within a limited space, as compared to conventional plain tubes. Returning to the topic of this post, the medium on the fin side is generally gas. For the fin side of outer-fin heat exchange tubes, without considering the perturbing effects of other components, when gas flows through the shell side of a plain-tube heat exchanger (assuming no baffle plates are used), as mentioned earlier, the value on the plain-tube side remains slightly higher than that on the fin side within a given volume; the magnitude of this difference is related to factors such as the structure of the outer-fin heat exchange tubes.

The use of fins reduces the heat transfer coefficient on one side of the fins, but it significantly increases the heat exchange area; the increased heat transfer amount **exceeds the decrease in heat transfer amount caused by the reduced heat transfer coefficient. For the overall heat transfer coefficient, the use of fins can increase it.

Essentially, it increases the convective heat transfer area.